The present invention relates capacitive transducers self-biased by a thin-film electret. More specifically, it relates to wide dynamic range force, pressure, and displacement sensors; electrostatic actuators; and acoustic and ultrasonic transceivers with multiple capacitor elements.
Simple electrostatic transducers comprise a variable capacitor with two, substantially parallel-plate electrodes: a flexible electrode responsive to a physical effect and a rigid counter-electrode. A change in pressure or force applied to the flexible electrode causes it to deflect. Displacement of the flexible electrode is electrically sensed by detecting a change in capacitance between cooperating electrodes. When a polarization voltage V0 is applied across the electrodes, the voltage vc(t) across the capacitor for a small diaphragm displacement xcex4(t) can be approximated by,                                           v            c                    ⁡                      (            t            )                          ≈                                            δ              ⁡                              (                t                )                                                    s              0                                ⁢                      V            0                          ≈                                            Δ              ⁢                              xe2x80x83                            ⁢                              C                ⁡                                  (                  t                  )                                                                    C              0                                ⁢                      V            0                    ⁢                      xe2x80x83                    ⁢                      (                          δ              ⁢                               less than  less than                                   s                  0                                                      )                                              (        1        )            
where C0 is the quiescent capacitance and s0 is the equilibrium spacing between the capacitor plates biased with a static potential Vo.
Conventional capacitive microphones utilize a high polarization voltage V0 to transduce the amplitude of a time varying acoustic pressure pa(t) to an open circuit voltage vc(t) across the capacitor electrodes. The polarization voltage is applied across the microphone capacitor through a high-value charging resistor R that maintains a substantially constant charge Qo=C0V0 on the variable capacitor. The sensitivity S of a microphone in terms of its open-circuit voltage divided by the pressure amplitude of an incident acoustic wave of angular frequency w can be expressed as                     S        =                                                            v                c                            ⁡                              (                t                )                                                    Δ              ⁢                              xe2x80x83                            ⁢                              p                ⁡                                  (                  t                  )                                                              =                                                    Δ                ⁢                                  xe2x80x83                                ⁢                                  C                  ⁡                                      (                    t                    )                                                  ⁢                                  V                  0                                                            C                0                                      ·                                          C                0                                                              C                  0                                +                                  C                  s                                                      ·                                          jω                ⁢                                  xe2x80x83                                ⁢                                  R                  ⁡                                      (                                                                  C                        0                                            +                                              C                        s                                                              )                                                                              1                +                                  jω                  ⁢                                      xe2x80x83                                    ⁢                                      R                    ⁡                                          (                                                                        C                          0                                                +                                                  C                          s                                                                    )                                                                                                                              (        2        )            
where Cs is the total parasitic capacitance including the input capacitance of amplification electronics. Eq. 2 can be further refined to include additional frequency dependent factors to account for specific electrode geometry and the effective mass and compliance of the diaphragm and surrounding fluids.
Electret capacitive transducers operate without an external dc polarization voltage. A thin-film electret is affixed to or formed on one capacitor electrode. An electret has a permanent state of electrical polarization that provides an electric field to self-bias a variable capacitor. This permits small electret microphones to be manufactured in high volume for hearing aids and communications equipment by avoiding the cost and complexity of providing a low-noise source of high voltage.
Many methods are known to provide electrets and electret capacitance microphones. Four references of note are: 1) G. M. Sessler and J. E. West, xe2x80x9cSelf-Biased Condenser Microphone with High Capacitance,xe2x80x9d Acoust. Soc. of Am. J. 34: 1787-1788, 1962; 2) U.S. Pat. No. 3,740,496 of Carlson et al.; U.S. Pat. No. 5,536,982 of Mino et al.; and 4) U.S. Pat. No. 2001/0033670 A1 of Tai et al.
Prior-art electret transducers are constructed with substantially parallel-plate electrodes. The sensitivity, linearity, and dynamic range of these capacitive transducers are limited by the geometric constraints of parallel-plate construction. The capacitance-displacement sensitivity of a gap-varying capacitor at mid-range signal frequencies is substantially:
xcex94C/xcex94s=xe2x88x92xcex5A/s2xe2x80x83xe2x80x83(3) 
The dependency on s2 results in a non-linear capacitance sensitivity with plate spacing.
Other disadvantages of prior-art, variable capacitors result from the minimum spacing that can be reliably maintained between parallel spaced electrodes and associated low values of quiescent capacitance. A transducer with low quiescent capacitance has a high source impedance 1/jxcfx89C at acoustic frequencies. This generally requires the capacitor voltage vc(t) to be detected by a JFET amplifier. The noise of a FET and high value bias resistors further limit dynamic range at low frequencies. Another disadvantage of small quiescent capacitance is a loss in sensitivity S due to stray capacitance. The total parasitic capacitance Cs of fringing fields, support structure, electrodes, and inputs of electronic circuitry reduces sensitivity S in Eq. 2 by the factor Co/(Co+Cs).
The spacing between capacitor electrodes limits the maximum displacement of a movable electrode. This displacement is further restricted by the well-known xe2x80x9cpull-inxe2x80x9d instability occurring at a critical voltage at which the movable electrode deflects by about ⅓ of the undeflected capacitor gap. Precision capacitance accelerometers use electrostatic force-rebalanced feedback to maintain an inertial mass suspended on a flexible electrode at a substantially fixed position to minimize non-linear capacitance response. However, feedback cannot significantly increase sensitivity or avoid the disadvantages of small quiescent capacitance.
The disadvantages of capacitance transducers with parallel electrodes (with or without an electret) are avoided by the variable-area capacitor embodiments of U.S. Pat. No. 6,151,967 and U.S. patent application Ser. Nos. 09/834,691 and 09/866,351.
A variable area capacitor (VAC) is referred to herein as a variable capacitor for which a substantial portion of a change in capacitance with electrode displacement is due to an increase in effective electrode area rather than to a change in electrode spacing. The capacitance of this type of VAC increases as an area of fixed capacitive spacing increases between cooperating electrodes while the approach of a movable electrode with respect to a stationary electrode remains small. This increases the effective area A contributing the majority of the capacitance between the electrodes and accommodates large displacements not limited by the dimensions of a narrow air gap. When a rising voltage is applied to a VAC, an electrostatic force of attraction continuously collapses a flexible electrode across a curved surface of a cooperating rigid electrode.
The large changes in capacitance of an electret VAC, up to 500% and more, can be linearly transduced by circuit inventions disclosed in U.S. patent application Ser. Nos. 09/794,198 and 09/816,551. An electret VAC with a thin flexible diaphragm can be operated as an electrostatic actuator or as an acoustic transmitter. When a variable voltage is applied across a VAC, acoustic or ultrasonic energy couples to the medium in which it is immersed.
The dynamic range of electret capacitive transducers can be extended several orders of magnitude using VAC""s with the general construction of the embodiments of U.S. Pat. No. 6,151,967. Electret transducers of the present invention can be constructed in part by methods disclosed in U.S. patent application Ser. No. 09/834,691. The instant invention can be advantageously applied to VAC transducers constructed to detect physical effects including force, pressure, acceleration, and displacement. VAC transducers can be operated as electrostatic actuators, with and without, force-rebalanced feedback control. All aspects of the present invention are applicable to sensors and actuators with multiple VAC elements.
Accordingly, the principle object of the present invention is to provide electret sensors and actuators with the low-noise, high capacitive sensitivity, and wide linear dynamic range characteristic of VAC transducers. This object is realized by electrically polarizing a thin dielectric film placed between a flexible electrode and a curved rigid counter-electrode. Non-limiting examples of VAC transducer embodiments in which an electret can be used to provide a self-biased transducer with a predetermined response characteristic are disclosed in U.S. Pat. No. 6,151,967 and U.S. patent application Ser. Nos. 09/834,691, 09/866,351, and 09/954,670.
More specifically, it is desirable to obtain large, linear changes of voltage vc(t) in Eq. 1 for values of xcex4so to provide high values of sensitivity S, as defined by Eq. 2.
Other objects of the present invention are to:
1. Provide electret VAC transducers with a flexible electrode comprising a simple, edge-supported cantilever beam;
2. Provide electret VAC transducers with a flexible electrode comprising a diaphragm;
3. Provide electret VAC transducers with multiple sensing and actuation elements electrically connected in parallel or alternately connected individually;
4. Provide electret VAC transducers with a response characteristic governed by a predetermined surface contour of a rigid electrode, such as a contour selected to maximize linear dynamic range;
5. Provide electret VAC transducers with high values of quiescent capacitance to avoid the noise limitations of small capacitors and detection electronics and a reduction in sensitivity due to parasitic capacitance;
6. Provide electret VAC transducers that employ electrostatic force-rebalanced feedback to sense and control the displacement of an inertial mass, stylus, or optical element.
7. Provide electret actuators that provide an electrostatic force to mechanically bias a mechanical, electrical, or optical element around a desired position of static equilibrium.
In accordance with the present invention, transducers including at least one VAC sensing or actuator element is self-biased by a thin electret film. The electret provides a DC polarization voltage across a flexible electrode and a stationary electrode with a surface contoured region. An electrical signal is generated that is substantially linearly proportional to displacement of the flexible sensing electrode over a wide dynamic range.
The above and other objects and advantages of the present invention will become apparent from consideration of the following description and drawings.